Add Angular Best Practices

A first crack at some Angular best practices arrived at through a seven
month angular project and discussions with other teams doing angular
work around the company. I'm no angular expert, but these are practices
I can remember working well, particularly in the face of other options
that *did not* work equally well.
Prefer `reduce` over `inject`

`reduce` provides symmetry with our already-preferred `map`.
Additionally, it has a more intention-revealing name and is more
approachable for people unfamiliar with it. "I am reducing this array to
something". `inject` doesn't make the same sense.
Prefer `if` over `unless`

Prefer e.g. `if signed_ou?` to `if ! signed_in?`

Rephrase, add example
Remove sample
Just prefer if over unless